make: define CONFIG_COMPAT on mrproper/clean

As it's a make/build option, the set of files to compile depends
on it. We don't do feature tests on mrproper/clean (no point in it).
Let's define CONFIG_COMPAT for those targets, so compiled files
will be properly cleaned.
Reported-by: Adrian Reber <adrian@lisas.de>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

Makefile

@@ -191,6 +191,9 @@ endif
 export CONFIG_HEADER := criu/include/config.h
 ifeq ($(filter clean mrproper,$(MAKECMDGOALS)),)
 include $(SRC_DIR)/Makefile.config
+else
+# To clean all files, enable make/build options here
+export CONFIG_COMPAT := y
 endif
 
 #